This invention relates to positioning of tubular workpieces, and in particular it relates to the correct positioning of a tubular parison on a carrier member associated with a parison heating oven.
In the art of blow molding, the starting material delivered to the blow mold is usually a hollow cylindrical tube knwon as a parison. Since the parisons must be hot when they are formed into finished articles, the original technique was to take the parison as soon as it was formed by the extrusion apparatus, and while still hot transfer it directly to the blow molding apparatus. However, since it has been found undesirable to necessarily link together the process of forming the parisons and the process of forming the finished articles, a further technique has been developed, known as the cold parison technique, wherein the parisons, upon being formed, are first of all cooled down and usually stored for a period of time after which they are reheated and blow molded. The separation of these two functions provides the significant advantage that the process of forming the parisons may be separated in time and geographically from the process of forming the finished articles. The separation of these two functions of course means that the process of forming the finished articles must start with cold parisons which must first be heated. Consequently, this latter technique has become known as the cold parison technique.
A method and apparatus for carrying out the cold parison technique is shown and described in commonly owned U.S. Pat. No. 3,765,813, issued Oct. 16, 1973. This patent illustrates an oven into which the parisons are placed and in which the parisons are heated to a forming temperature, after which the heated parisons are removed and carried to a blow mold, whereat the parisons are stretched longitudinally and then blow molded into finished articles.
As shown in the said patent, the parisons are loaded by gravity onto upwardly extending carrier members which then carry the parisons through the oven. It is of critical importance that the parisons be heated uniformly as they travel through the oven. Except for the case where non-uniformed heating is applied and carefully controlled for obtaining certain conditions, it is necessary that the parison be heated uniformly so that the subsequent stretching and blow molding steps will act upon the parison in a uniform manner. Otherwise, for example if the parison is not heated uniformly along its length or around its circumference, the finished article will have an improper weight distribution and hence will have to rejected. Obviously rejects constitute waste which is expensive and should be minimized if not totally eliminated.
Many methods and structural features are provided in the parison heating oven to achieve this required uniformity. For example the airflow through the oven is carefully controlled and the parisons are rotated about their axes.
However, these procedures designed to enhance heating uniformity presume in the first instance that the parison is at a specified position relative to its carrier member, i.e. that it is properly positioned on its carrier member. However, it has been found that with the present technique of loading the parisons onto their carrier members by dropping them vertically thereonto, many of the parisons have not in fact moved downwardly onto the intended, proper position on their respective holders. Moreover, with the presently known apparatus there is no further means for positively positioning a parison which did not achieve the proper position at the loading station. Consequently, many of the parisons remain in an improper position as they travel through the oven. For example, a parison might not fall downwardly far enough, whereupon it would sit high on its carrier member and perhaps tilt to one side. Such improperly positioned parisons would then continue through the oven and be heated in an unintentional, undesirable non-uniform manner, whereupon that parison would be stretched and blow molded in a non-uniform manner, resulting in a reject. Such improper positioning might be caused for example if the parison is slightly out of round, as a result of which the inside would be slightly oval, resulting in at least one short inside diameter preventing the complete downward movement of the parison onto its carrier member.
Previous attempt to assure proper positioning of parisons onto their respective carrier members have centered on tightening tolerances of the parison and the carrier member. However, the cost of very close tolerances are unreasonably high and they still cannot prevent an unreasonably high number of rejected parisons.
Thus, there is a need for an improvement which will economically and reliably assure that the parisons are properly positioned on their carrier members.